1. Field of the Invention
This invention relates to a capacitor microphone, and more particularly relates to a capacitor microphone which can efficiently prevent noises caused by outside electromagnetic waves and wind noises.
2. Description of the Related Art
Generally, a microphone unit for a capacitor microphone has high impedance, and includes an impedance transformer in order to reduce impedance of voice signals. The impedance-reduced voice signals are then transmitted to a low-cut circuit and an output circuit. In order to make a tiepin or headset type microphone less visible, a microphone unit usually houses only an impedance transformer while a low-cut circuit and an output circuit are housed in a circuit housing. The microphone unit and the circuit housing are connected by a dedicated cable.
The microphone unit is provided with an acoustic terminal via which sound waves are received. A nondirectional microphone needs one acoustic terminal while a unidirectional microphone of a primary sound pressure gradient type needs a pair of acoustic terminals at its front and rear ends. (Refer to Japanese Patent Laid-Open Publications No. 2000-232,700 and No. Hei 05-007,398).
FIG. 3 of the accompanying drawings shows a microphone unit 4 of a capacitor microphone of the related art. The microphone unit 4 is housed in a cylindrical case 2, and includes an acoustic terminal 10 at its front end. An acoustic resistor 8 made of a urethane sponge is housed in a front end of the cylindrical case 2, and has its one side in close contact with the acoustic terminal 10. A wire netting 6 is fixedly attached to the front end of the cylindrical case 2 in order to assure static shielding. The wire netting 6 is pressed in the shape of a cup having a flat bottom, and is fitted into the cylindrical case 2 with its straight sides 61 extending over the inner surface of the acoustic resistor 8. Referring to FIG. 4, the straight sides 61 of the wire netting 6 and the inner surface of the case 2 are brought into contact with one another at several points, thereby forming electric contacts 12. A rear peripheral edge of the cylindrical case 2 is crimped or is subject to the drawing compound in order to prevent the microphone unit 4 from dropping off. A cord (not shown) extends out of the rear end of the cylindrical case 2.
The acoustic terminal 10 of the microphone unit 4 is statically shielded by the wire netting 6 as shown in FIG. 3 and FIG. 4. However, the wire netting 6 is attached to the cylindrical case 2 using a rubber adhesive or the like, and is electrically connected to the cylindrical case 2 only via a plurality of points. This means that the electric connection is unreliable between the wire netting 6 and the cylindrical case 2, and that the static shielding also becomes unreliable. If outside electromagnetic waves arrive at an area where static shielding is unreliable, a high frequency current will flow through an internal circuit, be detected by a semiconductor element, and cause noises. Specifically, as cellular phones become popular and are more frequently used near a microphone, the microphone often suffers from noises caused by electric waves of the cellular phones.
Further, when airflows caused by winds or voices, especially airflows caused by plosive sounds, strike on the acoustic terminal 10 of the microphone unit 4, wind noises or pops will be produced. Usually, the acoustic resistor 8 is attached to the acoustic terminal 10, and is made of a fabric or a sponge, which has acoustic resistance but is non-conductive. If such an acoustic resistor 8 is used, the foregoing unreliable electric connection between the wire netting 6 and the cylindrical case 2 becomes further unstable, which will make the static shielding less reliable. This will cause more noises resulting from electromagnetic waves.